This invention relates to a method of operating a vapor generator system, in particular, operating a vapor generating system including a once-through vapor generator producing wet vapor at high loads and superheated vapor at low loads. More particularly, this invention relates to a method of operating the steam generating system of a steam-electric power station. Still more particularly, this invention relates to a method of operating a steam generating system, including a once-through steam generator, in a water-cooled nuclear reactor power station.
The vapor generating system of a power plant typically includes one or more vapor generators, a turbine, a condenser, a secondary coolant system and interconnecting piping. In water-cooled nuclear power stations, the vapor generators provide the interface between a reactor (primary) coolant system and the secondary coolant loop, that is, the vapor generating system. Heat generated by a reactor is transferred from the reactor coolant in the vapor generators to vaporize a secondary coolant, usually feedwater, and produce steam. The steam passes from the vapor generator to the turbine where some of its energy is used to drive the turbine. Steam exhausted from the turbine is condensed, regeneratively reheated, and pumped back to the vapor generators as feedwater.
In most pressurized water cooled nuclear steam supply systems, the steam exiting the vapor generators is routed directly to the turbine as dry or superheated steam. When once-through vapor generators are utilized, the steam is often superheated and provided at substantially constant pressure at the turbine throttle over the entire load range.
A typical once-through vapor generator employs a vertical, straight tube bundle, cylindrical shell design with shell side boiling. Hot reactor coolant enters the vapor generator through a top nozzle, flows downward through the tubes, wherein it transfers its heat, and exits through bottom nozzles before passing onto the reactor. The shell, the outside of the tubes, and the tubesheets form the vapor-producing section or secondary side of the vapor generator. On the secondary side, subcooled secondary coolant flows downward into an annulus between the interior of the shell and a tube bundle shroud, and enters the tube bundle near the lower tubesheet. As the secondary coolant flows upwardly through the tube bundle, heat is transferred from the counterflowing reactor coolant within the tubes, and a vapor and liquid mixture is generated on the secondary side ranging from zero quality at the lower tubesheet to substantially dry, one hundred percent quality vapor. The mixture becomes superheated in the upper portion of the tube bundle. The superheated vapor flows downwardly through an upper annulus between the shell and the tube bundle shroud, passes through a vapor outlet, and then onto the turbine. This arrangement insures zero moisture (superheated) vapor at the turbine throttle without the need of bulky steam drying equipment integrally associated with the vapor generators which, in nuclear power stations, are housed within a generally crowded environment in a reactor containment building where space is at a premium. Further detailed description of a once-through vapor generator may be found in U.S. Pat. No. 3,385,268.
The once-through vapor generating concept permits easily controlled operation with both constant average primary coolant temperature and constant steam pressure at the turbine throttle. To change load, the once-through vapor generator relies on a change in the proportion of boiling to superheating length in the tube bundle, that is, a trade-off between nucleate boiling and superheating. In designing and operating vapor generators, it is vital to make efficient use of the heat transfer surface. Hence, it is desirable to maintain nucleate boiling over as wide a range of vapor qualities as possible since nucleate boiling is characterized by high heat transfer coefficients and makes possible the generation of vapor with minimum heating surface. Typically, at high loads the once-through vapor generator heat transfer surface is approximately 75% in nucleate boiling and 25% in superheating; while at low loads the distribution is approximately 5% nucleate boiling and 95% superheating. Control is achieved by regulating feedwater flow to maintain constant output pressure, letting the distribution between superheating and boiling surface automatically vary as a function of load. One disadvantage of this concept is the relatively low heat transfer rate, or effectiveness, of the superheating surface at maximum load which requires more heating surface than would be needed if the heat were all transferred in the nucleate boiling mode. However, superheating is basically required to preclude moisture carry-over to the turbine, particularly during load change excursions.
Due to the single-pass, nonconcentrating characteristics of once-through vapor generators, essentially all of the soluble contaminants in the incoming secondary coolant exit from the unit dissolved in the superheated vapor, in moisture droplets that may be entrained and carried in suspension by slightly superheated vapor. In contrast, recirculating vapor generators concentrate solids in the feedfluid, and limit such concentrations by controlled blowdown. Hence, blowdown is not required in once-through vapor generators, but high quality secondary coolant is required.
In steam systems, feedwater is cleaned, for example, by condensate demineralizers prior to its introduction into the steam generator. Some contaminants remain in the feedwater regardless of the feedwater treatment utilized. Small quantities of common contaminants in feedwater chemistry can be tolerated and feedwater chemical specifications make appropriate allowances therefor. However, if the feedwater contaminants exceed limits allowed by the chemical specifications, either due to variations during normal operating conditions or during load transients, contaminants may be deposited within the turbine where corrosion damage can result due to the buildup and concentration of solids, particularly sodium compounds. Allowable sodium concentrations may be as low as 1 ppb. Unfortunately, a greater proportion of sodium compounds to total solids seems to be present when condensate polishing is used.
Thus, there exists a need to develop operating techniques for vapor generating systems including once-through vapor generators which further minimize contaminant deposition in the turbine and which minimize the disadvantages of utilizing steam generator heat transfer surface for superheating.